An embodiment of a sensor for determining a presence of a substance in a gaseous sample is known from U.S. Pat. No. 5,281,915. The known sensor is arranged for analyzing an incoming gas flow for detecting a presence of a substance having a high molecular weight, for example a presence of a substance having molecular weight of about 100 or higher. For this purpose the known sensor is arranged with an ionization region arranged for producing ionized gas molecules, said ionization region comprising a radiation source disposed in a chamber arranged under atmospheric pressure, said chamber cooperating with a further region wherein the ionized oxygen and nitrogen molecules undergo secondary ionization to produce heavy ions. This further region is adapted with electrodes for inducing a potential difference in the region for allowing heavy ions to float towards a collector electrode whereat these ions recombine producing an electrical signal representative of the level of high molecular weight contamination in the inflowing gas. In order to ensure an adequate detection sensitivity of the high molecule contamination, the known sensor comprises a heater for heating the inflowing gas to a predetermined temperature.
It is a disadvantage of the known sensor that a two-step gas ionization is used, whereby a radioactive source is utilized for causing primary ionization of the light molecules. In addition, the known sensor is only operable under atmospheric pressure due to its design features, pertaining to an ion drift caused by the application of an external electric field. These operational conditions may cause an unallowable recombination rate of ions within the sensor influencing its accuracy. Next, in order to provide a sensible measurement the inflowing gas must be heated, which may limit application areas of such sensor.